1. Field of the Invention
This invention concerns the field of bonding together multiple layers to form a laminate and, more particularly, a method of bonding wherein an adhesive impregnated scrim is preheated and then placed between the layers of material to be bonded.
2. Description of the Relevant Prior Art
Bonding of laminates by using a film of adhesive that activates under heat to become either tacky or liquid and flowable to form the bond is well known in the prior art. The resultant heat activated bond is mechanical or chemical, or both. The technique of heat activating a film of adhesive is used in many industrial applications and, particularly, in situations where the resultant laminate is molded to form such items as interior panels on vehicles. Examples of patents dealing with the lamination of multiple layers using some form of thermally activated adhesion between the layers are U.S. Pat. Nos: 3,996,082; 4,221,619; 4,500,594; 4,571,279; 4,588,458; 4,711,681; and 4,731,276.
All of the above referenced patents are similar in that the adhesive is heated after it is placed between the layers to be laminated; in other words, the entire laminate sandwich structure must be heated to activate the adhesive. Hence, the activation temperature of the adhesive must, necessarily, be low enough that the heating will cause no harm to any of the materials to be laminated. Furthermore, this prior art technology has other shortcomings. There can be problems getting enough heat through the laminates (which are often formed of thermally insulating materials) to melt the adhesive layer. A typical method of bonding is the use of hot dies (molds and platens); which need much expensive energy to become sufficiently hot. The dies are expensive, and the cycle times are, typically, quite long. These long cycle times result in high capital investment per unit of capacity.
For contoured parts, the laminates are molded by use of heated dies. However, this results in high stress areas on the contoured parts. To mold these highly stressed parts, they must be held in the die until cool down (for a thermoplastic material) or thermoset (for a thermosetting material) occurs. The necessity of cooling down the highly contoured parts causes excessive dwell time in the expensive dies, further exacerbating the problems noted in the prior paragraph. Some systems inject a cooling gas into the die, but such dies must be designed to withstand the stresses caused by the temperature cycling, and are, hence, even more expensive. Furthermore, highly contoured parts cannot be efficiently done on a production basis with current adhesive films due to these problems.
The combination of the viscosity of the molten adhesive combined with the long, hot dwell cycles under pressure in the prior art often causes migration of the molten adhesive into porous substrates. This migration results in adhesive starvation at bond lines and resultant poor bonds in reproducible and non-reproducible modes, and is also detrimental to the porous substrate.
The large molds necessary to form large items have inherent problems of hot and cold spots; temperature variations of 25.degree. F. from one part of the platen to another are normal. Furthermore, pressure differences from one section of the platen to another are also normal due to size variations in the substrate layers and die misalignment. These conditions magnify the problems noted in the preceding paragraphs.
It is known to heat the substrates inside the dies by injection of superheated dry steam. While this system offers a solution to some of the problems noted above, such as migration, a new complication is introduced to the system; the injected superheated dry steam must be kept clean.
Furthermore, in the prior art, the melt temperature of the adhesive must be low enough not to cause damage to the substrates. This requirement imposes a severe constraint on both the types of substrates that may be used, as well as the adhesive itself.